research review Theorists question Auger as the primary cause of LED droop
BAND–TO-BAND Auger recombination does not account for droop, the decline in nitride LED efficiency at increasing drive currents. That’s the claim of a team of theorists based in the US and Italy, which is at odds with previous calculations of Chris Van de Walle’s group at the University of California, Santa Barbara. Last year these West-coast academics performed first-principles, density-functional electronic structure simulations on GaN and InN, and used these results to determine that Auger recombination was a likely cause of LED droop.
These calculations, like those of the US- Italian team, considered the Auger coefficient for non-radiative processes involving two electrons and one hole at a range of bandgaps.
Van De Walle’s team found that the strongest Auger process revolves around resonant electron scattering from the lowest to the second lowest conduction band. This Auger coefficient peaks at 2 x 10-30
cm6 s-1
for InGaN with a bandgap of 2.5 eV. In contrast, perturbation theory calculations by the three-man team from Boston University and the Politecnico di Torino, Italy, indicate that the Auger coefficient for the resonant interband coefficient is far, far lower – less than 10-32
cm6 at a higher energy, 2.8 eV. s-1 . And it occurs
In addition, this triumvirate of Francesco Bertazzi, Michele Goano and Enrico Bellotti has calculated the Auger coefficient for processes involving two holes and one electron. This process has no resonance peak, steadily decreases as the bandgap increases, and is less than 10-32
cm6 s-1 for
an InGaN bandgap greater than 2 eV. The US-Italian team has tried to fathom why its value for the Auger resonance peak is 0.3 eV higher than that provided by UCSB .
They believe that this discrepancy stems from employing different values for the energies of the lowest and second lowest conduction bands in InGaN. They began by adopting a ‘nonlocal empirical pseudopotential method’ to calculate the band structure for InN and GaN, an
III-V MOSFETs scale successfully
As channel MOSFET with a sub-50 nm gate. Realizing a device on this length scale is a key step towards the development of III-V transistors that are credible alternatives to those produced with state-of- the-art silicon CMOS processes.
Engineers from The University of Texas at Austin claim to have produced the first In0.7
Ga0.3
The 40 nm gate-length device made by the US team has an impressive set of attributes: a current drive of 507 mA/mm at a gate voltage of 1 V, an intrinsic transconductance of 1475 mS/mm, an on-off ratio of 1 x 105
,
and a sub-threshold swing 132 mV/dec at a source-drain voltage of 50 mV.
The engineers have focused their efforts on a buried channel device. This is claimed to be better at maintaining high mobility than surface channel equivalents, thanks to the separation of the channel from the
Buried channel MOSFETs were made by isolating mesas and removing the cap in the gate region by wet etching with citric acid. Atomic layer deposition added a 5 nm-thick layer of Al2
O3
was added to form the gate electrode. E- beam lithography and reactive ion etching defined the shape of this gate, before e-
46
www.compoundsemiconductor.net January / February 2011 onto the III-V surface, and TaN
The team believes that it should also be possible to realize better gate control by either shrinking the oxide thickness or turning to a dielectric with a higher κ value, a step that will also compensate for short channel effects.
F. Xue et al. (2010) Electron. Lett. 46 1694
oxide/III-V interface. According to the team, this class of device is also able to realize a far lower gate leakage current than HEMTs if a high-κ gate dielectric is employed.
MBE was used to produce MOSFET epistructures on 3-inch semi-insulating InP. They comprised a 300 nm-thick In0.52 buffer; a 10 nm quantum well In0.7
Ga0.3 Al0.48
Al0.48 As
of InP; and a 20 nm thick, heavily doped InGaAs cap.
As and a 2 nm-thick layer As
channel; a double barrier with a 1.5 nm-thick layer of In0.52
beam evaporation of palladium and germanium created source and drain ohmic contacts.
One of the weaknesses of the MOSFET is its low extrinsic conductance of 570 mS/mm, which stems from a relatively high external resistance. This is a result of the large source-to-drain separation – it is about 3 µm – which can be reduced by employing a self-aligned process, a step that is necessary for future CMOS applications.
approach that allows parameters to be tweaked so that the electronic structure can replicate the main features obtained by either experiment or first principles calculations. Application of what is described as a modified virtual crystal approximation yielded the electronic structure for the conduction and valence bands in InGaN.
In comparison, the UCSB team first determined the energy difference between the lowest and second lowest conduction bands in InN and GaN, before linearly extrapolating values for InGaN.
The US-Italian team says that the difference between the value of the energy of the resonance peak calculated by them and the UCSB team impacts the calculation of the Auger coefficient. However, its impact is not large enough to account for the three orders of magnitude difference between the two calculations.
According to them, this massive difference could stem from the West coast team’s failure to fully account for the symmetry of the electronic states that are involved in the calculation of the Auger recombination strength in InGaN alloys.
F. Bertazzi et al. (2010) Appl. Phys. Lett. 97 231118
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